Process for the application of a metal film on a polymer surface of a subject

ABSTRACT

A process for the application of a metal film on a polymer surface of a subject, to which, after nucleation with a catalytically active platinum metal and/or a platinum metal compound, a coating metal is precipitated autocatalytically from an aqueous solution without any external current supply, before at least another layer is applied, preferably electrolytically under external current supply, to this preliminary coating. To achieve favorable construction features, it is suggested that centers of a Lewis acid or Lewis base are enriched first on the polymer surface to be coated by a vacuum plasma treatment via a plasma gas adapted to the polymer used, before the polymer surface is nucleated under vacuum with the platinum metal and/or the platinum metal compound by another plasma gas containing an evaporated platinum metal and/or a platinum metal compound for nucleophilic and/or electrophilic reaction on the Lewis acids and/or Lewis bases.

FIELD OF THE INVENTION

The invention relates to a process for the application of a metal filmon a polymer surface of a subject, to which, after nucleation with acatalytically active platinum metal and/or a platinum metal compound, acoating metal is precipitated autocatalytically from an aqueous solutionwithout any external current supply, before at least one further layeris applied, preferably electrolytically under external current supply,to this preliminary coating.

DESCRIPTION OF THE PRIOR ART

Plastic components, which provide benefits with respect to theirspecific gravity and in terms of simple shaping compared with metalcomponents, are metallised because of their outer appearance, on the onehand, and to influence their surface properties, particularly in termsof electric conductivity, mechanical properties or chemical resistance,on the other hand. However, this is difficult in case of subjects with apolymer surface, as the polymer surface of the subject to be coated mustbe pretreated by complex processes, before the metal coating can beapplied electrolytically with sufficient adhesion. As it is, the polymersurface to be coated is first nucleated with noble metals, in generalplatinum metals, i.e. elements of the 8^(th) sub-group of the periodsystem (Ru, Rh, Pd, Os, Ir, Pt), and/or compounds thereof, to then beable to realise premetallisation via an autocatalytic precipitation of acoating metal from an aqueous solution without external current supply.Nucleation of the polymer surface with a platinum metal requires apreliminary wet-chemical treatment in pickling and activation bathsconsisting of solutions of strong and frequently concentrated acids andlyes as well as toxic compounds. For example, in a conventional processthe polymer surface is pre-pickled, after cleaning with alcohol and/oran organic solvent, in a bath of chromic-sulfuric acid mixture atincreased temperature, to be then subjected to the pickling processproper in chromic-sulfuric acid mixture at increased temperature. Nextto the preliminary treatment with an acid, the polymer surface can benucleated, after neutralisation of the pickled polymer surface, with aplatinum metal from an appropriate platinum metal solution. By means ofthe described multi-stage preliminary treatment of the polymer surface agood adhesion of the platinum metal nuclei at the polymer surface shallbe ensured as a prerequisite for an appropriate adhesion of thepremetallising cost, to which a thicker metal coating is subsequentlyapplied in a conventional way by electrolytic precipitation of thecoating metal under external current supply. The drawback of this knownmetallisation of polymer surfaces is not only the comparatively complexprocess, but also the fact that waste water containing heavy metals isgenerated during preliminary treatment of the polymer surface. Moreover,in the area of the pickling and activation baths toxic vapours are to beexpected, and on top of that a considerable demand of chemicals must betaken into account.

SUMMARY OF THE INVENTION

It is therefore the objective of the invention to avoid these drawbacksand to design a process for the application of a metal film on a polymersurface of a subject of the above mentioned kind in such a way that anucleation of the polymer surface with platinum metals and/or compoundsthereof can be ensured without pickling and activation baths in solventsof strong acids and lyes, and without having to accept a reduction ofadhesion between the platinum metal nuclei and the polymer surface.

The objective of the invention is achieved in that centers of a Lewisacid or Lewis base are first enriched with a plasma gas adapted to thepolymer used in a vacuum treatment, before the polymer surface isnucleated under vacuum, with the platinum metal and/or the platinummetal compound via another plasma gas containing an evaporated platinummetal and/or a platinum metal compound for the nucleophilic and/orelectrophilic reaction on the Lewis acids and/or Lewis bases.

By means of the suggested plasma treatment of the polymer surface notonly the surface tension of the polymer is reduced in a principallyknown way, and a good wettability of the subject surface to be coated isachieved, but also a chemical reaction in the surface area is triggeredvia the formation of centers of Lewis acids or Lewis bases, according tothe plasma gas used. The incidental polarisation of the polymer surfaceis a fundamental prerequisite for a bonding connection between thepolymer surface and the metal ions. As Lewis acids and Lewis basescorresponding with each other in strength show a preferential reactionon each other, the polymer surface coated with centers of a Lewis acidor Lewis base is subjected, in another step of conditioning, to a plasmatreatment with an evaporated platinum metal and/or a platinum metalcompound, with this plasma gas constituting a corresponding Lewis baseand/or Lewis acid, so that nucleophilic and/or electrophilic reactionsare achieved with the effect that the platinum metal atoms and/or themolecules of the platinum metal compound are durably bound to thepolymer surface without losing their catalytic efficiency for thesubsequent electroless electrolytic premetallisation.

For the formation of the Lewis acids or Lewis bases on the polymersurface various gases may be used depending on the polymer to be coatedin each case. To obtain centers of Lewis bases and/or Lewis acids on thepolymer surface in a favorable way, the applied plasma gas may containoxygen or an oxygenous compound such as carbon oxides (CO, CO₂) or watervapor. By means of the oxygen of the plasma gas the polymer molecules atthe surface are partly oxygenated in such a way that new hydroxy,carbonyl and carboxy groups are formed, which, due to their polarity,have a strong affinity to the metal ions introduced by the subsequentplasma treatment.

Another favorable way to polarise the polymer surface is the use of aplasma gas with a sulfurous compound for the formation of Lewis acids orLewis bases. Such sulfurous compounds like sulfur oxides (SO₂, SO₃),sulfur fluorides (SF₆) or hydrosulfide (H₂S) lead, in case of a plasmaexcitation according to the kind of polymer, to a sulfonation of, e.g.,aromatic compounds and thus to the desired polarisation of the polymersurface. Other gases reacting on the polymer surface may be used aswell, namely nitrogenous compounds such as nitrogen oxides (NO, NO₂,N₂O) or ammonia (NH₃), or halogens or hydrohalogens.

Due to the two-stage plasma treatment, which may be performedconsecutively in the same treatment tank, but also in treatment tanksindependent from each other, the complex preliminary wet-chemicaltreatments in pickling and activation baths are avoided, so that thedrawbacks related to these preliminary wet-chemical treatments areavoided, too. Nevertheless, a good adhesion is achieved between theplatinum metals and the polymer surface, which is indispensable forensuring a permanent metallisation of the polymer surface.Premetallisation can be done in the conventional way in that the metalprovided for precoating is precipitated autocatalytically without anyexternal current supply from the aqueous electrolyte on the polymersurface nucleated with platinum metals. The thickness of thisautocatalytic film is chosen such that it is an thin as possible, butsufficiently free from pores, so that the subsequent galvanicprecipitation of the metal film proper is not impaired. The thickness ofthe autocatalytically precipitated metal layer is approx. 0.2-2μ,preferably 1μ. Onto this preliminary coating a thicker metal film of thethickness demanded in each case can be precipitated galvanically underexternal current supply. However, this preliminary coating may also becoated with layers by way of other coating techniques, for instance ifthese layers do not permit a galvanic precipitation, or if the latter isnot desired.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following examples of embodiment a square flat piece of polymerwith a side length of 80 mm and a thickness of 5 mm was put, after usualpre-cleaning, into a treatment tank connected to a vacuum pump, fromwhich the air was evacuated up to a negative pressure of 0.1 mbar,before plasma gas for polarising the polymer surface was passed into thetreatment tank, which, at a treatment pressure between 5 and 10 mbar,was excited to ignation of the plasma by means of high frequency. Duringa preset treatment time the polymer surface was partially chemicallychanged by the corresponding reaction on the plasma gas, so that centersof Lewis acids and/or Lewis bases were generated. After this treatmentof the flat pieces of polymer the plasma gas was evacuated at a negativepressure of 0.1 mbar, before another plasma gas was passed in underevaporation of a platinum metal compound at a pressure of 1 mbar. At atreatment pressure set between 1 and 5 mbar the plasma was ignited againby means of external excitement via radiofrequency, so that, during thesubsequent treatment period, the centers of Lewis acids and/or bases onthe polymer surface could react on the platinum metal compounds which infact have some properties of Lewis bases and/or Lewis acids. Afterventilation of the treatment tank the flat piece of polymer nucleatedwith a platinum metal compound on the surface was taken out of thetreatment tank and provided with a preliminary metal coat in anautocatalytically active electrolyte without external current supply,with the thickness of the preliminary metal coat being approx. 1 μm.Onto this preliminary metal coat different layers could be applied in aconventional way, with an altogether excellent adhesion of all layers,achieved due to the good adhesion of the platinum metal compounds on thepolymer surface.

EXAMPLE OF EMBODIMENT 1

The flat piece of plastic to be coated consisted of anacrylonitrile-butadiene-styrene-copolymer, whose surface was treatedwith nitrogen oxide (N₂O) as the plasma gas for the formation of strongLewis bases, for a treatment period of 60 a. Palladium nitrate was usedas the platinum metal compound. The carrier gas used was argon. After atreatment period of 30 s for nucleation with the palladium nitrate, apreliminary coat was applied in a nickeliferous, autocatalyticelectrolyte, where nickel with 5% in weight phosphorus was precipitatedwithout any external current supply. Onto this preliminary coat a layerof bright copper of a thickness of 50 μm was galvanically applied underexternal current supply in the principally known way.

EXAMPLE OF EMBODIMENT 2

Instead of the acrylonitrile-butadiene-styrene-copolymer of the aboveexample of embodiment a flat piece of plastic made ofpolybutyleneterephthalate was used, which was first subjected to aplasma treatment with a plasma gas composed of half oxygen and halfsulfur dioxide for the formation of centers of strong Lewis acids on thepiece surface during a treatment period of 480 s. As the platinum metalcompound forming a strong Lewis basetetrakis-(-triphenylphosphine)-palladium was chosen, namely inconnection with a carrier gas of argon with 5% by volume of hydrogen.The nucleation time was 480 s. Similar to the example of embodiment 1, alayer of nickel with 2% in weight phosphorus was precipitated withoutcurrent supply from an electrolyte to the polymer surface nucleated withthe chosen platinum metal compound. The final galvanic coat, however,was replaced by a coat of lacquer of a two-pot transparent lacquer witha thickness of layer of 5 μm.

EXAMPLE OF EMBODIMENT 3

Finally, a flat piece of polyethenimide was subjected, under appropriateconditions, to a treatment with a plasma gas of nitrogen with 20% byvolume of hydrosulfide, which led to centers of weak Lewis bases on thepolymer surface (treatment period 180 s). For the formation of weakLewis acids allyl palladium chloride was evaporated in a carrier gas ofargon, to which ammonia was added, and used for further plasma treatmentof the piece of plastic for a treatment period of 120 s. This time, thepreliminary coating was applied electrolessly with copper. Metallisationproper was achieved by galvanic precipitation of bright copper underexternal current supply at a layer of thickness of 50 μm. Instead ofsuch a galvanically precipitated layer, a layer may also be appliedphysically in a vacuum, for instance a hard material of titanium nitride(thickness of layer e.g. 1 μm).

What is claimed is:
 1. A process for the application of a metal film ona polymer surface comprising the steps of: (a) enrichment of Lewis acidsor Lewis bases on said polymer surface by means of a vacuum plasmatreatment, said vacuum plasma treatment comprising a plasma gas adaptedto said polymer; (b) nucleation under vacuum with a catalytically activeplatinum metal and/or platinum metal compound by means of a secondplasma treatment, said second plasma treatment comprising a secondplasma gas containing an evaporated form of said platinum metal and/orplatinum metal compound for nucleophilic and/or electrophilic reactionon said Lewis acids or said Lewis bases; (c) autocatalytic precipitationof a coating of a metal from an aqueous solution without any externalcurrent supply; and (d) electrolytically applying at least anothercoating of metal, wherein said plasma gas used for the enrichment ofsaid Lewis acids or said Lewis bases on said polymer surface contains ahalogen or a hydrohalogen.
 2. The process of claim 1, wherein said atleast another coating of a metal is applied under external currentsupply.
 3. A process for the application of a metal film on a polymersurface comprising the steps of: (a) enrichment of Lewis acids or Lewisbases on said polymer surface by means of a vacuum plasma treatment,said vacuum plasma treatment comprising a plasma gas adapted to saidpolymer; (b) nucleation under vacuum with a catalytically activeplatinum metal and/or platinum metal compound by means of a secondplasma treatment, said second plasma treatment comprising a secondplasma gas containing an evaporated form of said platinum metal and/or aplatinum metal compound for nucleophilic and/or electrophilic reactionon said Lewis acids or said Lewis bases; (c) autocatalytic precipitationof a coating of a metal from an aqueous solution without any externalcurrent supply; and (d) electrolytically applying at least anothercoating of a metal, wherein said plasma gas used for the enrichment ofsaid Lewis acids or said Lewis bases on said polymer surface contains asulfurous compound.
 4. Process according to claim 3, wherein said plasmagas used for the enrichment of said Lewis acids or said Lewis bases onsaid polymer surface contains ammonia.
 5. The process of claim 3,wherein said at least another coating of a metal is applied underexternal current supply.
 6. A process for the application of a metalfilm on a polymer surface comprising the steps of: (a) enrichment ofLewis acids or Lewis bases on said polymer surface by means of a vacuumplasma treatment, said vacuum plasma treatment comprising a plasma gasadapted to said polymer; (b) nucleation under vacuum with acatalytically active platinum metal and/or platinum metal compound bymeans of a second plasma treatment, said second plasma treatmentcomprising a second plasma gas containing an evaporated form of saidplatinum metal and/or a platinum metal compound for nucleophilic and/orelectrophilic reaction on said Lewis acids or said Lewis bases; (c)autocatalytic precipitation of a coating of a metal from an aqueoussolution without any external current supply; and (d) electrolyticallyapplying at least another coating of a metal, wherein said plasma gasused for the enrichment of said Lewis acids or said Lewis bases on saidpolymer surface contains ammonia.
 7. The process of claim 6, whereinsaid at least another coating of a metal is applied under externalcurrent supply.